This invention is directed to a method of rejuvenating silicoaluminophosphate (SAPO) molecular sieve catalyst, and a method of using the rejuvenated catalyst to make an olefin product from methanol feed. In particular, the invention is directed to rejuvenating the sieve by contacting the molecular sieve with anhydrous, polar liquid or vapor until a desired methanol uptake ratio is achieved.
Silicoaluminophosphates (SAPOs) have been used as adsorbents and catalysts. As catalysts, SAPOs have been used in processes such as fluid catalytic cracking, hydrocracking, isomerization, oligomerization, the conversion of alcohols or ethers, and the alkylation of aromatics. In particular, the use of SAPOs in converting alcohols or ethers to olefin products, particularly ethylene and propylene, is becoming of greater interest for large scale, commercial production facilities.
As is known in the development of new large scale, commercial production facilities in the commodity chemical business, many problems arise in the scale up from laboratory and pilot plant operations. Scale up problems arise in catalytic reaction systems where large scale operation will be several orders of magnitude larger than typical pilot scale facilities. For example, conventional laboratory scale processes of making olefin products from oxygenate feed are conducted with catalyst loads of about 5 grams. Conventional large pilot plant operations may utilize as much as 50 kg of catalyst, making on the order of 20 kg/hr ethylene and propylene product, but this is nevertheless minuscule in comparison to what a large scale, commercial production facility would produce, if one were in existence today. Large scale, commercial production facilities, can require a catalyst loading of anywhere from 1,000 kg to 700,000 kg, producing anywhere from 600 to 400,000 kg/hr of ethylene and propylene product.
Operating large scale, commercial production facilities clearly presents great challenges in the development of the catalyst production-to-use chain. The term xe2x80x9cproduction-to-use chainxe2x80x9d refers to the entire area of activities beginning with the production of molecular sieve, including such activities as receipt of starting materials, on through the crystallization process. Also included in the production-to-use chain are intermediate activities which include formulation of the sieve with binders and other materials, activation of the manufactured sieve and finished catalyst; storage, transport, loading, unloading of molecular sieve and finished catalyst; as well as other practices associated with the handling and preparation of the sieve and finished catalyst for its ultimate use. The production-to-use chain ends at the point when the molecular sieve is introduced into the reaction system. For purposes of this invention, the end of the production-to-use chain does not necessarily mean the instant when the molecular sieve is introduced into the reaction system, since large scale systems are very large and instantaneous measurements are not practically feasible. In large scale systems, the production-to-use chain may be considered as completed some time within 12 hours of loading catalyst into the reaction system.
Since information to date relating to production of olefin products by catalytic conversion of oxygenate feedstock has been limited to laboratory and small pilot plant activities, little if any attention has been paid to the problems associated with the intermediate activities in the production-to-use chain. For example, little attention has been focused on the impact of storage, transport, etc. on catalyst activity, since small scale activity is rather easily manageable. While today only relatively small quantities of catalyst are stored and transported, large quantities of materials will need to be handled for commercial operations. Commercial operations may be required to store large quantities of sieve and catalyst materials for considerable periods of time, at multiple locations, and under rather rigorous industrial conditions.
As the management of sieve and catalyst in the catalyst production-to-use chain expands in volume and complexity, a likelihood exists that millions of dollars will be tied up in catalyst inventory, and the value of the sieve and catalyst will be lost if quality is not maintained at every step. Loss of quality will necessarily translate to loss of product quality, as well as loss of product quantity, and these product losses could far outweigh the cost of the sieve and catalyst.
Although some work has been published relating to the intermediate activities in the catalyst production-to-use chain, few of the problems associated therewith have been addressed. For example, U.S. Pat. No. 4,681,864 to Edwards et. al. discuss the use of SAPO-37 molecular sieve as a commercial cracking catalyst. It is disclosed that activated SAPO-37 molecular sieve has poor stability, and that stability can be improved by using a particular activation process. In this process, organic template is removed from the core structure of the sieve just prior to contacting with feed to be cracked. The process calls for subjecting the sieve to a temperature of 400-800xc2x0 C. within the catalytic cracking unit.
U.S. Pat. No. 5,185,310 to Degnan et al. discloses another method of activating silicoaluminophosphate molecular sieve compositions. The method calls for contacting a crystalline silicoaluminophosphate with gel alumina and water, and thereafter heating the mixture to at least 425xc2x0 C. The heating process is first carried out in the presence of an oxygen depleted gas, and then in the presence of an oxidizing gas. The object of the heating process is to enhance the acid activity of the catalyst. The acid activity is enhanced as a result of the intimate contact between the alumina and the sieve.
Briend et al., J. Phys. Chem. 1995, 99, 8270-8276, teach that SAPO-34 loses its crystallinity when the template has been removed from the sieve and the de-templated, activated sieve has been exposed to air. Data are presented, however, which suggest that over at least the short term, this crystallinity loss is reversible. Even over a period of perhaps two years, the data suggest that crystallinity loss is reversible when certain templates are used.
EP-A2-0 203 005 also discusses the use of SAPO-37 molecular sieve in a zeolite catalyst composite as a commercial cracking catalyst. According to the document, if the organic template is retained in the SAPO-37 molecular sieve until a catalyst composite containing zeolite and the SAPO-37 molecular sieve is activated during use, and if thereafter the catalyst is maintained under conditions wherein exposure to moisture is minimized, the crystalline structure of the SAPO-37 zeolite composite remains stable.
A group of researchers at ExxonMobil Chemical Company has recently discovered that activated SAPO molecular sieve will exhibit a loss of catalytic activity when exposed to a moisture-containing environment. This loss of activity can occur between the time the catalyst is activated and even after as little as one day of storage. Although ways have been found to inhibit loss of catalytic activity, it would be highly beneficial to find a way to reverse the decrease of catalytic activity in a molecular sieve exposed to a moisture-containing environment.
In order to overcome the various problems associated with decrease of activity of a molecular sieve due to contact by moisture, this invention provides a way to reverse such decrease, i.e., to rejuvenate the molecular sieve. In general, this invention provides a process for rejuvenating a molecular sieve which comprises providing a molecular sieve having a methanol uptake index of less than 1; and contacting the molecular sieve with anhydrous liquid or vapor until the methanol uptake ratio is increased by at least 10%.
Preferably, the molecular sieve is a silicoaluminophosphate molecular sieve and it is provided having a methanol uptake index of less than 0.5, more preferably a methanol uptake index of less than 0.3, and most preferably, a methanol uptake index of less than 0.15. In another preferred embodiment, the methanol uptake ratio is increased by at least 50%, more preferably by at least 100%, and most preferably by at least 500%.
It is also desirable that the molecular sieve be contacted with anhydrous liquid or vapor until a methanol uptake ratio of at least 0.4 is achieved, preferably at least 0.6, more preferably at least 0.7, and most preferably at least 0.8.
In another preferred embodiment of the invention, the anhydrous liquid or vapor is polar. Desirably, the anhydrous liquid or vapor contains not greater than about 30 wt. % water, preferably not greater than about 20 wt. % water, and more preferably not greater than about 10 wt. % water. It is also desirable that the anhydrous liquid or vapor have a kinetic diameter of not greater than 1.5 times the average pore size of the molecular sieve, preferably not greater than 1.3 times the average pore size. It is further desirable that the anhydrous liquid or vapor have a gas-phase proton affinity greater than or equal to the proton affinity of water.
Desirably, the anhydrous liquid or vapor is an alcohol, ether, ketone, carboxylic acid, aldehyde, nitrogen containing organic bases or mixtures thereof. Preferably, the anhydrous liquid or vapor is selected from the group consisting of methanol, ethanol, dimethyl ether, propylamine, and acetonitrile. More preferably, the anhydrous liquid or vapor is methanol.
The silicoaluminophosphate molecular sieve is preferably selected from the group consisting of SAPO-5, SAPO-8, SAPO-11, SAPO-16, SAPO-17, SAPO-18, SAPO-20, SAPO-31, SAPO-34, SAPO-35, SAPO-36, SAPO-37, SAPO-40, SAPO-41, SAPO-42, SAPO-44, SAPO-47, SAPO-56, the metal containing forms thereof, and mixtures thereof. Preferably, the silicoaluminophosphate is selected from the group consisting of SAPO-18, SAPO-34, SAPO-35, SAPO-44, SAPO-47, the metal containing forms thereof, and mixtures thereof. More preferably, the silicoaluminophosphate is selected from the group consisting of SAPO-18 and SAPO-34, the metal containing forms thereof, and mixtures thereof.
In an alternative embodiment, the invention includes a method of making an olefin product from an oxygenate-containing feedstock. The method comprises forming a rejuvenated molecular sieve; and contacting the rejuvenated molecular sieve with an oxygenate-containing feedstock to produce an olefin product.
Desirably, the oxygenate-containing feedstock is selected from the group consisting of methanol; ethanol; n-propanol; isopropanol; C4-C20 alcohols; methyl ethyl ether; dimethyl ether; diethyl ether; di-isopropyl ether; formaldehyde; dimethyl carbonate; dimethyl ketone; acetic acid; and mixtures thereof. Preferably, the oxygenate-containing feedstock is selected from the group consisting of methanol, dimethyl ether, and mixtures thereof.
It is also desirable that, in the method of making the olefin product, the rejuvenated molecular sieve is contacted with the oxygenate-containing feedstock at a temperature of 200xc2x0 C. to 700xc2x0 C. Preferably the rejuvenated molecular sieve is contacted with the oxygenate-containing feedstock at a WHSV of at least 20 hrxe2x88x921. It is also preferred that the silicoaluminophosphate molecular sieve is provided in catalyst form, i.e., with a binder material.
The invention also provides contacting the olefin product a polyolefin-forming catalyst under conditions effective to form a polyolefin. The preferred olefin product contains ethylene and/or propylene, which can be used to form polyethylene and/or polypropylene. The olefin and polyolefin products so formed are also considered to be encompassed by the invention.